I'd like to hear some opinions on the idea of a Vim spell that, while in effect, negates the penalties for using no Voice or Gestures on spells cast subsequently.
First off, should such a spell be allowed?
If so, how would you design it? If not, why not?
Personally, I'm on the fence about the idea, leaning toward 'allow'. It doesn't really negate the value of the Quiet or Subtle Magic virtues, as they work automatically on any spell of any level you cast. However, it might depreciate the value of the Quiet and Still Casting Mastery abilities, slightly, though the Mastery abilities have the advantage of not requiring another spell to be cast in order to gain the benefit, and they work on any level of spell.
I don't see anything wrong with the general idea. I think you could argue that perhaps it is allowable with the existing general MuVi guideline to "Significantly change" a spell (which is what powers Wizard's Boost). Or perhaps, for your saga it is a new guideline.
However, I wouldn't implement like you have. I'd make it much more like Wizard's Boost, (i.e. needs to be cast on each spell, rather than cast on the wizard) and require a separate spell for removing the Voice and Gestures penalty. Definitely should be Form specific (and explicitly exclusive to Hermetic magic), but shouldn't need a requisite. So, something like:
Quiet Voice of Aquam
R: Touch, D: Momentary, T: Individual
This spell is cast as another Hermetic Formulaic or Ritual spell, (of Aquam Form), of a level less than this one is cast. The caster of the Aquam spell recieves no penalties for not using his voice for the casting. Equivalent spells are possible for other Forms. Separate, similar spells may remove the penalty for casting without Gestures.
Facilitate the Stifled (Form) Spell
R: Touch, D: Mom, T: Ind
This spell is cast at the same time as another formulaic spell (ArM5, page 159), whose level must be less than twice the level of this spell. If successfully cast, the targeted spell may be cast with no penalty to the casting total due to reduced or absent gestures and/or voice. While the Muto Vim spell needs gestures and voice as normal, the casting total of the follow-up spell is not penalized, and thus difficult spells at the limit of a magus's ability have a greater chance of success when in restricted circumstances, without incurring fatigue loss. If the targeted spell has Range Voice, the volume used to cast the Muto Vim spell determines the actual range. There are ten versions of this spell, each affecting spells of one of the Hermetic Forms. This spell is particularly appropriate to Bjornaer magi who wish to maintain the appearance of being a mundane animal, yet still cast spells.
(Base effect, +1 Touch)
Note that there is an error in that MuVi spells are not necessarily cast at the same time as the spell that they affect. It is useful because a magus might manage a -15 penality to cast a 15th level version of this spell, which then enables them to cast a 30th level spell with no voice or gestures. They might not be able to manage the 30th level spell at a -15 penalty. It might also be useful to preserve Penetratin Totals in covert situations - e.g. if cast with quiet voice and no gestures, which gives a -10 penalty to the casting of the Vim spell, but not to the follow up spell.
The spell can't be done as you designed it (with D:Sun), because the Target of the MuVi spell (that is, spells you might choose to cast over the next <12 hours) cannot be sensed yet.